1. Field of the Invention
The present invention relates to the thermal treatment of biomass. It has particular, but not exclusive, application to biomass pyrolysis reformers for the production of renewable gaseous and solid fuels from biomass.
2. Related Art
Biomass pyrolysis is the thermal decomposition of biomass (e.g. plant material such as wood and wood bark) substantially in the absence of oxygen. Biomass is typically a mixture of hemicellulose, cellulose, lignin and small amounts of other organics. These components typically pyrolyse or degrade at different rates and by different mechanisms and pathways.
One traditional example of biomass pyrolysis is the production of charcoal, where the main product of the pyrolysis is char. Alternative biomass pyrolysis techniques provide a product which, after cooling, includes a substantial proportion of liquid. This liquid is typically a dark brown liquid having a heating value that is around one half the heating value of conventional fuel oil. The liquid is typically referred to as bio-oil. In some circumstances, it is the bio-oil which is the most valuable product of the pyrolysis reaction, since bio-oil can be easily stored for later use, e.g. for heat and/or electricity generation. However, in other circumstances, the gas products may be more useful, e.g. in rural locations for combined heat and power (CHP) applications where the gas may be used to produce electricity.
The rate and extent of decomposition of the components of biomass depends on the process parameters of the pyrolysis reactor. In turn, these process parameters may also have an effect on the subsequent behaviour of the product, e.g. by secondary reactions such as cracking (of higher molecular mass products) or condensation reactions (of lower molecular mass products).
In order to produce a high proportion of gas phase by a pyrolysis process, it is typical to carry out a gasification-type pyrolysis process. In such a gasification process, it is typical to heat the solid biomass to 300-600° C. to achieve pyrolysis of the biomass, the products of which are solid char, condensable organic compounds (including tar), water and gases. Subsequently in the process, reactions are promoted above about 700° C. (typically at around 800° C.) to decrease the liquid (vapour) concentration to produce further useful gas phase products, and also to gasify some of the char via gas-solid and gas-gas interactions. There are several different types of gasifier reactor types that have been characterised. See, for example, A. V. Bridgwater (“Renewable fuels and chemicals by thermal processing of biomass” Chemical Engineering Journal Volume 91, Issues 2-3, 15 Mar. 2003, pages 87-102), which disclosure briefly reviews the characteristics of the following gasifier reactor types: downdraft fixed bed, updraft fixed bed, bubbling fluid bed, circulating fluid bed, entrained flow, twin fluid bed, screw/auger kiln, rotary kiln, cyclonic and vortex.
In each of the gasifier reactor types indicated above, it is necessary to provide a heat transfer medium in order to achieve rapid and efficient heating of the biomass feedstock and the products of pyrolysis to promote gasification. Typically particles such as sand are used as the heat transfer medium.
WO 02/50484 discloses an apparatus for the thermal treatment of material. It is primarily intended for the recycling of electronics waste material, but can also be used for the thermal treatment of biomass. WO 02/50484 discloses a screw kiln in which thermally conductive particles are provided in the feedstock. These particles are spheres of metal, ceramics or SiC. Primarily they have the function of cleaning the interior surface of the screw kiln. On exiting the kiln, the thermally conductive particles may be re-used by being conducted back to the entrance to the kiln along a hollow shaft of the kiln.